This invention relates to sheet shaping and more particularly to the fabrication of articles from thermoplastic sheet materials. The shaping or forming of hot thermoplastic sheets to desired shapes is currently done by methods which are either "mechanical" or "pneumatic" wherein a mechanical process involves the use of a solid mold or tool either moving or stationary whereas a pneumatic process involves the use of differential air pressure created either by vacuum or compression. One well-known article formed by such methods are containers for comestibles.
In the thermoforming process for producing such containers, a sheet of thermoplastic material either upon reheating or still sufficiently heated as it emerges from the extruder is contacted by a large wheel-like device having a plurality of molds positioned about the periphery thereof and adapted to contact the sheet and by differential pressure as by the application of vacuum to the center of the wheel enables the sheet to be drawn downwardly upon the mold so as to form sets of individual containers and a nonformed interconnecting material web. As the large wheel rotates, the sheet slowly cools to a point where it is self-supporting whereupon it is withdrawn and the individual containers cut or trimmed from the traveling web or sheet of material. Thus, in order to provide adequate time in which the sheet may become self-supporting after being shaped, it is necessary to provide a rather large peripheral travel distance for such sheet; and, hence, the equipment for such purposes may be necessarily large.
It would, however, be desirable to produce articles such as the above-indicated containers without the need for utilizing such large and accordingly necessarily expensive equipment. Thus, if the time required for the shaped sheet to become self-supporting could be reduced, then the size of such attendant equipment could also be reduced.
Attention is directed to a recently introduced and unique in-die strand cooling process disclosed in U.S. Pat. No. 3,981,959 and issued Sept. 21, 1976, to the assignee of the present application, namely, Leesona Corporation. In such patent, a system which permits drastically increased pelletizing operation efficiencies by means of the introduction of a liquid coolant under pressure into direct contact with the outer surface of a polymeric or other material passing through it, is disclosed. As the coolant enters the die orifice, a portion thereof is immediately vaporized to form at least a solidified outer wall of material surrounding an inner and still somewhat liquid material core. The wall of the orifice is simultaneously lubricated by nonvaporized coolant to facilitate its passage through the die. Such system, available from Cumberland Engineering Company, Inc., Providence, Rhode Island, also contemplates, although not necessarily so, the subsequent cutting of the resultant strands into discrete material lengths or pellets by knives mounted for rotation in respect to the exit face of the die.
The above cited U.S. patent and its discussion constitutes applicant's Prior Art Statement.